1. Field of the Invention
The present invention relates to a hybrid DC electromagnetic contactor, and in particular to a hybrid DC electromagnetic contactor capable of preventing arc occurrence in opening/closing of a hybrid-structured contactor and minimizing a leakage current by connecting a semiconductor switch to a mechanical contact switch in parallel.
2. Description of the Prior Art
In general, an electromagnetic contactor or an electromagnetic switch is used for connecting/cutting off power and load electrically.
The contactor connects/cuts off separately fixed-installed two electrodes through a moving electrode, power of an electromagnet is used in connecting, and power of a spring is used in cutting off (separating). Herein, when the switch is open and a current flows into the electrode, because arc is generated at a contact point due to energy stored in a stray inductance element of a line or a load or a power side, the contact point may be damaged.
Accordingly, a specific material and shape is required for the contact point of the contactor in order to stand the arc occurrence. And, in order to extinguish the arc instantly and safely, an arc extinguishing portion having a certain shape is required at the upper end of the contact point of the contactor.
In order to overcome the problem of the mechanical electromagnetic contactor, a SSR (solid-state relay) or a SSC (solid-state contactor) which replaces mechanical contact points of an AC electromagnetic switch with semiconductor switches has been presented and used. However, because lots of heat is generated by an applied current due to a voltage lowering at both ends of the semiconductor switch, an additional heat sink or cooling fan is required, and accordingly they have been used only for special purposes.
In addition, it is also possible to replace a DC electromagnetic contactor with a semiconductor switching device having a forced extinguishing function, however a mechanical DC electromagnetic contactor has been mainly used still.
FIG. 1 is a circuit diagram illustrating a construction of a conventional AC hybrid breaking switch.
As depicted in FIG. 1, AC power 1 is connected/separated to/from a load 7 through a mechanical main contact point 5. In a general AC electromagnetic switch, a sub contact point 4 is installed as a basic unit.
However, in the conventional AC hybrid switch, the main contact point 5 is connected in parallel to a triac 2 that functions as a two-way semiconductor switch, a resistance 3 is connected between a gate terminal G and an anode terminal A of the triac 2, and the sub contact point 4 of the switch is connected between the gate terminal G and a cathode terminal K of the triac 2.
The basic operation of the conventional AC hybrid breaking switch will be described through state changes (open or closed state) of the main contact point 5 of the switch.
In the breaking switch, when the main contact point 5 is open, the sub contact point 4 is closed, the gate G of the triac 2 is short-circuited from the cathode K, and the triac 2 maintains an off state. Herein, minute current (several tens˜several hundreds mA) flows between the AC power 1 and the load 7 through the resistance 3.
In order to turn on the switch, when a voltage is applied to a coil 6, the main contact point 5 and the sub contact point 4 are moved, first the sub contact point 4 is opened before the main contact point 5 is closed, an operation signal is applied between the gate G and the cathode K of the triac 2, and accordingly several tens˜several hundreds current flows into the gate terminal G of the triac 2.
Herein, because the triac 2 operates regardless of polarity of a gate current, it is turned on only when sufficient gate current flows into the triac 2, the AC power 1 and the load 7 are connected to the triac 2, and accordingly a current flowing on the load 7 flows into the triac 2.
When the main contact point 5 is closed after a certain time has passed, a chattering phenomenon occurs due to mechanical characteristics, a current flows on the gate G of the triac 2 in opening of the main contact point 5, and accordingly an arc is not generated at the mechanical contact point.
When the mechanical contact point is closed completely, both end voltages of the triac 2 reach almost to 0, a minimum voltage (in general, several volts) required for turning-on the triac 2 is not secured, and accordingly the triac 2 is turned off.
Afterward, when the voltage applied to the coil 6 is removed in order to turn off the switch, the moving electrode part of the main contact point 5 and the sub contact point 4 is moved, and the main contact point 5 is open first.
In opening of the main contact point 5, the current flows again on the gate G of the triac 2, the triac 2 is turned on, and the load current flows. Herein, because voltage lowering at both ends of the triac 2 is not greater than several volts, the generation of an arc is restrained.
After a certain time has passed, when the sub contact point 4 is closed, the gate G and the cathode K of the triac 2 are short circuited, the current flow on the gate G is 0, the polarity of the current flowing through the triac 2 is changed, and the load current continually flows through the triac 2 until the triac 2 is turned off.
However, the hybrid switch in FIG. 1 can be applied only when power is AC, if power is DC, because there is no method for extinguishing the triac 2 as a semiconductor switch device, a power semiconductor switching device having a forced extinguishing function such as an IGBT (insulated gate bipolar transistor), a MOS-FET (metal oxide semiconductor-field effect transistor) and a BJT (bipolar junction transistor) has to be used.
Hereinafter, a DC hybrid contactor using the IGBT will be described.
FIG. 2 is a circuit diagram illustrating a construction of the conventional DC hybrid contactor.
As depicted in FIG. 2, DC power 13 is connected/separated to/from a load 12 through a mechanical main contact point 14.
A semiconductor switch unit 11 is connected to the main contact point 14 in parallel, and the ends of a diode Df are connected to the load 12 and a—terminal of the DC power 13.
The semiconductor switch unit 11 includes an IGBT switch QA, a free wheeling diode Df, a snubber diode DS1, a snubber capacitor CS1 and a snubber resistance RS1.
The operation of the conventional DC hybrid contactor is similar to that of the AC hybrid contactor in FIG. 1.
When the open state of the main contact point 14 is changed to the closed state, an arc occurs due to a chattering phenomenon caused by mechanical characteristics. However, because a size of the arc is small, it is possible to turn off the IGBT switch QA in the region, and accordingly only changing the closed state of the main contact point 14 into the open state will be described. Herein, in controlling the IGBT switch QA, if the load is a capacitor, a large in-rush current occurs when the switch is turned on. In that case, a current value flowing on the IGBT switch device is too big, and a production cost of the switch may rise.
First, in the opening state of the main contact point 14, because the IGBT switch QA is turned off, the DC power 13 and the load 12 are connected with each other through the snubber circuits DS1, CS1, RS1. Accordingly, in order to turn on the contactor, a voltage is applied to a coil 19, herein, the IGBT switch QA maintains the turn-off signal applied state.
In order to turn off the turned-on switch, the semiconductor switch QA connected to the mechanical contactor in parallel is turned on first, the voltage applied to the coil 19 is removed, the current flowing through the main contact point 14 flows through the semiconductor switch QA, the voltage on both ends of the turned-on semiconductor switch QA is 2V ˜3V, and the main contact point 14 can be opened without any arc occurring. After a certain time has passed, when the operation signal applied to the gate G of the semiconductor switch QA is removed, the current flowing through the load 12 flows through the diode Df and the resistance RS1 and is stopped. Afterward, energy stored in a stray inductance Lw of the DC power side is absorbed into capacitorCS1, the current flowing through the semiconductor switch QA is stopped, and accordingly, the turn-off process of the contactor is finished.
In the conventional hybrid contactor, when both the semiconductor switch QA and the main contact point are turned off, there is a problem. In more detail, in that state, the capacitor CS1 maintains a charged state with a voltage almost equal to the voltage of the DC power 13 or the turned-off state unless there is no voltage change (in particular, voltage increase) of the DC power 13.
However, the capacitor CS1 is actually charged due to the snubber discharge resistance RS1 when the voltage of both ends of the capacitor CS1 is smaller than the voltage of the DC power 13, the current flows from the DC power 13 to the load through the diode DS1, the capacitor CS1 and the resistance R s. Herein, when a resistance RS1 value is small, a large current flows, and when a resistance (RS1) value is large, a small current flows. If, the turn on/off processes are not performed frequently, it is possible to reduce a leakage current value by increasing a resistance (RS1) value sufficiently.
However, because the snubber circuit is for restraining a spike voltage on both ends of the switch in turning-off of the semiconductor switch QA, the resistance RS1 can not be increased that much. Accordingly, there is no way to prevent the leakage current phenomenon. In order to remove the leakage current, an additional switch for stopping discharge of the capacitor CS1 can be installed.
However, although the additional switch is installed, when a size of the power voltage 13 is changed according to the passage of time, there is no way to prevent the leakage current. If the DC power is a storage battery, the storage battery is discharged continually due to the leakage current. If a voltage of the DC power 13 is not less than 100V, there is a risk of an electric shock accident at the load block due to the leakage current.
In addition, in the conventional art, if a polarity of the power connected to the switch is changed or the connection between the power side and the load side is changed, the operation of the switch may not be performed at all.